Selectivity in Catalysis - American Chemical Society

Chapter 15

Reactions of meta-Diisopropylbenzene on Acid Molecular Sieves 1

1

1

Man-Hoe Kim , Cong-Yan Chen , and Mark E. Davis

Downloaded by UNIV LAVAL on October 27, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0517.ch015

Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Meta-diisopropylbenzene is reacted with propylene over the acid form of the molecular sieves SAPO-5, mordenite, offretite, beta, hexagonal and cubic faujasite (EMT and FAU), L, SAPO-37, and an amorphous silica -alumina at temperatures around 463 Κ in aflow-typefixed-bed reactor. A small amount of cracking is observed. However, the main reactions of meta-diisopropylbenzene are isomerization and alkylation. It is proposed that this alkylation can be used as a new test reaction to characterize the effective size of the voids in larger pore (12 T-atom rings or above) molecular sieves by measuring the weight ratio of 1,3,5- to 1,2,4 -triisopropylbenzene formed. In most cases, this ratio increases with the increasing effective void size of the molecular sieves in the order: SAPO-5 < mordenite < offretite < beta < EMT FAU < L < SAPO-37 < amorphous silica-alumina. Zeolites have been widely used as shape selective catalysts for a great variety of processes in the refining and petroleum industry (1). The discovery of the zeolite-like phosphate-based molecular sieves now extends the possibilities for shape selective catalysts (2). Notable phosphate-containing molecular sieves are the extra-large pore aluminophosphates VPI-5 (3) (one-dimensional 18-ring channels) and A I P O 4 - 8 (4) (one-dimensional 14-ring channels) and the very new gallophosphate cloverite (5,6) (supercage of 30 Â diameter and three-dimensional 20-ring pore openings). Recent efforts in the synthesis of new molecular sieves and their modification have resulted in a very large number of molecular sieve materials with vastly different pore sizes and void structures. For example, in Figure 1 the structures of the molecular sieves SAPO-5, offretite, mordenite, L , beta, cubic and hexagonal faujasite, and SAPO-37 are schematically illustrated. It is apparent that the tetrahedral units of S1O4/2, AIO4/2 and PO4/2 connect in different ways to form crystalline structures with cavities and/or channels of different size and shape. Although techniques such as H R E M (High Resolution Electron Microscopy) (7), M A S - N M R (Magic Angle Spinning NMR) (8), Neutron Diffraction^, etc. can be used to gain insight into the void structure of the 1

Current address: Chemical Engineering, California Institute of Technology, Pasadena, CA 91125

0097-6156/93/0517-0222$06.00/0 © 1993 American Chemical Society

In Selectivity in Catalysis; Davis, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


15. KIM ET AL.

Reactions of meta-Diisopropylbenzene

SAPO-5

FflU/SHPO-37

Beta (polymorph fl)

EMT

Figure 1. Schematic structures of molecular sieves.


223


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SELECTIVITY IN CATALYSIS

molecular sieve materials, they can rarely be applied at conditions suitable for catalytic reactions to occur. The sorption of molecules with systematically increasing diameters can also be used to characterize the pore structure of molecular sieve materials (10). However, this technique is subject to temperature regions where catalysis is not occurring and can only provide information on total pore volume and effective pore size. (Variation in effective pore size with changing temperatures has been clearly established.) The use of a catalytic test reaction is another method to determine the effective pore and/or void space sizes of molecular sieve materials (11). A new test reaction must first be tested using known molecular sieve materials with different pore structures to establish a correlation between the shape selectivities and the effective pore/void sizes. Subsequently, the reaction can be applied to molecular sieve materials with unknown crystal structures in order to formulate an estimate of their effective pore structures on the basis of shape selectivities. For the characterization of acid zeolites, workers at Mobil Research Laboratories have suggested the determination of the Constraint Index CI (12). This index involves the relative cracking rate of n-hexane and 3-methylpentane and is used for probing the effective pore size of 10-membered ring zeolites. Other acid-catalyzed test reactions are the conversion of alkylaromatics, e.g., meta-xylene (13,14), ethylbenzene (15), methylethylbenzene (16). In addition to acid-catalyzed reactions, bifunctional molecular sieves (metal particles in addition to acid sites) have been tested. The Refined Constraint Index CI* (17,18) and the Spaciousness Index SI (19,20) have found success in characterizing the effective pore size of 10- and 12-membered ring molecular sieves, respectively. The Refined Constraint Index CI* is based on the bifunctional hydrocracking and isomerization of n-decane and the Spaciousness Index SI is based on the bifunctional hydrocracking of Cio-naphthenes. In this paper, a new test reaction is suggested for characterizing the effective pore size of larger pore (12-membered rings or above) molecular sieve materials. Metadiisopropylbenzene is alkylated with propylene and the weight ratio of 1,3,5- to 1,2,4triisopropylbenzene formed is calculated. This ratio is used to characterize the effective void space size of molecular sieves. Experimental Catalyst Preparation. The molecular sieves used in this study were either purchased from commercial suppliers (Union Carbide: zeolites mordenite and L ) or obtained by hydrothermal synthesis according to the information published in the literature (offretite, beta, cubic and hexagonal faujasite, SAPO-5, and SAPO-37). The hexagonal polytype of faujasite (21,22) is designated E M T as assigned by the IZA structure commission. The as-made samples were washed with deionized water, recovered by filtration, dried at 383 Κ and subsequently calcined in air at 873 Κ for 6 hours (except SAPO-37) to remove the organic species occluded in the pore systems. The resulting materials were then transformed into their NH4+-forms by an ionexchange with a 1 M aqueous solution of N H 4 C I . The H+-forms of the catalysts (except SAPO-37) were obtained by activating their NH4+-forms in-situ in the reactor with a helium flow of 5 L/h at 623 K . The as-made SAPO-37 which does not contain any alkali-cations was calcined in-situ in the reactor in an oxygen flow of 6 L/h at 793 Κ for 10 hours in order to obtain its Η+-form without any observable losses in structural integrity. The amorphous silica-alumina (87 wt.-% silica and 13 wt.-% alumina) was purchased from Strem. To obtain the H+-form of the silica-alumina, it was pretreated in the same way as the molecular sieve catalysts studied here (except SAPO-37).


15. KIM ET A L


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Adsorption Measurement. The capacities of the molecular sieves to adsorb vapor phase 1,3,5-triisopropylbenzene (97 %, Aldrich) and 1,2,4-triisopropylbenzene (99%, Carnegie-Mellon University) were measured at 373 Κ using a McBain-Bakr balance. The adsorption temperature was chosen such that no chemical reactions of the adsorbates were observed. Prior to the adsorption experiment, the NH4+-forms of the solids (except SAPO-37) were dehydrated at 573 Κ under a vacuum of 10-2 Ton*. The as-made SAPO-37 was calcined at 793 Κ in an oxygen flow of 6 L/h in-situ in the adsorption system for removal of organic species and dehydration. The vapor pressure at 296 Κ of 1,3,5- and 1,2,4-triisopropylbenzene is approximately 0.45 Torr. The adsorption experiments were conducted at this pressure. Ammonia T P D Measurement. The acidic properties of the catalysts were characterized using temperature programmed desorption (TPD) of ammonia. The experiments were carried out on aflow-typeapparatus equipped with a fixed-bed and a thermal conductivity detector. The samples were activated in a helium flow of 5 L/h at 773 Κ for 1 hour. 300 mg of the H+-form of each dehydrated sample were used to perform the ammonia TPD. Pure ammonia, with a flow rate of 3 L/h, was then passed through the sample at 423 Κ for 30 min. The sample was subsequently purged with helium at the same temperature for 1.5 hours in order to remove the physisorbed ammonia. The TPD was performed under a helium flow of 6 L/h from 423 Κ to 873 Κ with a heating rate of 10 K/min and subsequently at the final temperature for 30 min. Catalytic Experiment. The alkylation of meta-diisopropylbenzene with propylene was performed at 463 Κ in a flow-type fixed-bed reactor. The carrier gas nitrogen was first saturated with the vapor of meta-diisopropylbenzene (97 %, Aldrich) and then admixed with propylene (99 %, Matheson). The partial pressure of propylene and meta-diisopropylbenzene was 42.6 and 6.0 Torr, respectively. (The molar ratio of propylene and meta-diisopropylbenzene at the reactor inlet was 7.1 : 1). The modified residence time of propylene and meta-diisopropylbenzene W/F opyi ne and W/F _oiPB ranged from 4 to 20 and from 25 to 150 gh/mol, respectively, where W indicates the weight of dehydrated catalyst at 623 Κ and F i indicates the molar flow rate of reactant i at the reactor inlet. The reaction conditions, viz. the reaction temperature, amount of catalyst, partial pressure and modified residence time of reactants, were chosen in order to obtain conversions of meta-diisopropylbenzene around 25 %. The products were analyzed with an on-line gas chromatograph using a 1.8 m packed column that contained 5 % SP-1200 and 75 % Ben tone 34 as the stationary phase. The saturator and reactor system have been described in greater details elsewhere (23). pr

e

m

Results and Discussion Adsorption Capacities. The adsorption capacities of the various molecular sieves and the amorphous silica-alumina for 1,3,5- and 1,2,4-triisopropylbenzene are listed in Table I. 1,3,5-Triisopropylbenzene does not adsorb into the channels of the acid form of SAPO-5, mordenite and offretite at an adsorption temperature of 373 K. The small adsorption capacity of around 0.005 g/g most likely is indicative of some surface and/or intercrystalline adsorption of 1,3,5-triisopropylbenzene in these three samples. On the other hand, all the other samples studied in this work possess cavities and/or channels large enough to accommodate 1,3,5-triisopropylbenzene. In addition, all the sieves used here are capable of adsorbing 1,2,4-triisopropylbenzene. It is obvious that the samples exhibit observable differences in adsorption capacity as a consequence of their different pore structures. These effects can be nicely


226


Table I. Adsorption data for 1,2,4- and 1,3,5-triisopropylbenzene at 373 K* g/g solid

Sample


1,2,4-TIPB

^1,3,5-TIPB

1,3,5-TIPB

^1,2,4-TIPB

SAPO-5

0.045

0.006

0.13

Mordenite

0.039

0.005

0.13

Offretite

0.039

0.007

0.18

Beta

0.129

0.052

0.40

EMT

0.179

0.115

0.64

FAU

0.141

0.143

1.01

L

0.032

0.027

0.84

SAPO-37

0.243

0.200

0.82

0.028

0.026

0.93

Si0 -Al 0 2

2

3

*Pressure of adsorbate is the vapor pressure at room temperature illustrated by calculating the adsorption capacity ratio W 1,3,5-TIPBAVΙ,2,4-ΉΡΒ as shown in Table I. Interestingly, this ratio ranges between 0.64 and 1.01 for the molecular sieves L, EMT, F A U , SAPO-37 and the amorphous silica-alumina, i.e., those solids that can adsorb both triisopropylbenzene isomers. The ratio of 0.40 for zeolite beta and 0.64 for EMT is unexpected and may upon further study reveal interesting features attributable to their unique structures. TPD Results. Figure 2 shows the results of ammonia TPD from the H+-form of the molecular sieves SAPO-37, L , F A U , EMT, mordenite, beta, SAPO-5 and the amorphous silica-alumina. Two different types of acid sites are distinguished which are related to the maximum ammonia desorption rates in the temperature region of 540 610 Κ and at ca. 770 K, respectively. The total amount of acid sites correlates to the area integrated below the ammonia TPD curve. The ammonia TPD curves of H - F A U and H-EMT illustrate that these two samples possess very similar acidity, which is in good agreement with the fact that they have the same Si/Al ratio of 3.4. The tailings of their TPD curves after the peak maxima are indicative of the presence of some strong acid sites the peaks of which are not resolved in these experiments. For SAPO-37, the narrow TPD profile reveals that this sample has slightly stronger acid sites than the majority of those in H - F A U and H-EMT. On the other hand, according to their TPD peak areas these three samples have very similar total amount of acid sites. For H-L, Η-beta and H-mordenite, there is some NH3 desorption near 770 Κ that indicates the presence of strong acid sites. However, in contrast to H-L and Η-beta, H-mordenite has a greater number density of strong acid sites than weak ones. Finally, for H-SAPO-5 and H-S1O2/AI2O3, only weak acid sites of small amount are observed.




15. KIM ET A L


227


228


Reactions of meta-diisopropylbenzene. The adsorption data given above clearly reveal that within the series of 12-ring molecular sieves studied here, 1,2,4triisopropylbenzene can be distinguished from 1,3,5-triisopropylbenzene. Also, it is known that at room temperature, VPI-5 adsorbs 1,3,5-triisopropylbenzene while NaY does not (equilibrium reached in several days) (24). Thus, it is speculated that the alkylation of meta-diisopropylbenzene with propylene to form triisopropylbenzenes may be useful for characterizing the size of the void spaces in molecular sieves with 12-ring pores or larger (ZSM-5 does not sorb and react meta-diisopropylbenzene at the conditions employed here). The premise is that as the void space increases in size the catalyst will produce a greater amount of 1,3,5-triisopropylbenzene since it is the thermodynamically preferred product. Also, it is important to point out that the reactions of diisopropylbenzene are very facile in that they do not require strong acid sites. This may be helpful when attempting to characterize phosphate-based molecular sieves (in general they have weaker acid sites than zeolites). Also, differences in acid site strength may not affect the selectivity results and thus allow direct comparison of a large number of materials with varying acid strength distributions. A problem with monofunctional reactions, e.g., cracking, alkylation, etc. is that they have a tendency to quickly deactivate because of coke deposition. This problem is usually not of concern with bifunctional reactions, e.g., those that employ a metal function in addition to the acid sites. However, we avoided the use of metal function because of the possible unknown modifications that could be introduced to a given sample by the metal deposition procedure. This is especially important when dealing with samples like VPI-5. Thus, to minimize the rate of deactivation, the alkylation experiments were conducted at 463 K . This low temperature introduces another problem, namely, the adsorption of reactants and products. At the experimental conditions employed here, the catalyst bed becomes saturated at time of 10 minutes or less (depending on sample). From this point onward, deactivation is clearly observable via the decrease in conversion with time. The data reported here were obtained at 11-13 minutes on-line. Since meta-diisopropylbenzene proceeds through several reaction pathways that lead to a number of products, it is most appropriate to compare the catalytic data at the constant level of conversion. Here we report selectivities at approximately 25 % conversion. For each catalyst, the results near 25 % conversion were repeated three times to ensure reproducibility. Table Π shows the results obtained from contacting meta-diisopropylbenzene and propylene with the various catalysts at 463 K . For all the catalysts except SAPO-5, the amount of cracked products is low (included in column labelled others). In addition, no ortho-diisopropylbenzene and trace 1,2,3-triisopropylbenzene were observed. Two trends are illustrated by the data in Table II. First, the ratio A/I of alkylated products (1,3,5- and 1,2,4-TIPB) to that formed by isomerization (p-DIPB) increases roughly as the size of the pore/void spaces increase. The exception is EMT. For E M T a very high isomerization rate is observed. However, it is interesting to note that E M T has a lower than expected W I ^ T I P B / W I ^ - T I P B adsorption capacity ratio which may be correlated to the catalytic results. Second, the ratio of 1,3,5- to 1,2,4triisopropylbenzene (1,3,5/1,2,4-TIPB) follows the trends in A/I with increasing void space size. Clearly these ratios (especially 1,3,5/1,2,4-TIPB) are an outcome of the catalyst structure. When isomerization competes with alkylation, then there is the possibility that some of the isomerized diisopropylbenzene (para) is alkylated as well. In order to test for this possibility, two other experiments were performed. At higher conversion, a greater amount of products formed by secondary reactions should be observable. Additionally, reactions with para-diisopropylbenzene will reveal the presence of secondary reaction if 1,3,5-triisopropylbenzene is formed (assuming that at lower conversion the small amount of 1,2,4-triisopropylbenzene will not produce a significant amount of 1,3,5-triisopropylbenzene by isomerization). Table ΠΙ gives the


15. KIM ET A L

229

Reactions of meiSL-Diisopropylbenzene

Table II. Comparison between the pore size of catalysts and the product distributions of meta-diisopropylbenzene alkylation with propylene at 463 Κ


1,3,5-TIPB

Conv.

Yield, mol-%

mol-%

p-DIPB 1,3,5- 1,2,4TIPB TIPB

SAPO-5

24.3

74.9

2.0

8.9

14.2

0.2

0.15

Mordenite

25.6

62.3

17.8

15.6

4.3

1.1

0.83

2.8

1.5

0.54

Catalyst

Offretite

25.1

Beta EMT

Others

A/I*

1,2,4-TIPB

53.2

26.4

17.6

24.2

55.4

25.1

14.3

5.2

1.8

0.71

25.3

42.0

35.0

15.4

7.6

2.3

1.20

FAU

27.4

21.0

54.0

21.5

3.5

2.5

3.60

L

25.5

23.8

53.4

18.5

4.3

2.9

3.02

SAPO-37

26.2

21.9

55.5

18.3

4.3

3.0

3.37

S1O2-AI2O3

24.2

17.9

62.1

17.9

2.1

3.5

4.47

*A/I: total alkylated products/total isomerized products results of these experiments performed on SAPO-5 and zeolite L . Notice that 1,3,5triisopropylbenzene is formed from a para-diisopropylbenzene feed in both cases. Thus, the secondary reactions of isomerized diisopropylbenzenes do contribute to the formation of triisopropylbenzenes. These results clearly demonstrate that the isomerization and alkylation processes (both primary and secondary) must be considered when attempting to rationalize the product distributions. Table III. Reaction of diisopropylbenzenes over SAPO-5 and zeolite L

Catalyst

Reactant Τ, Κ

Conv. mol-%

1,3,5-TIPB

Yield, mol-% DIPB

1,3,5- 1,2,4TIPB TIPB

1,2,4-TIPB

0.2

SAPO-5

m-DIPB

463

24.3

SAPO-5

m-DIPB

473

46.0

64.0(p)

5.0

15.0

0.3

SAPO-5

p-DIPB

473

32.0

47.0(m)

6.0

28.0

0.2

L

m-DIPB

463

25.5

23.8(p)

53.4

18.5

2.9

L

m-DIPB

473

51.0

35.0(p)

46.0

14.0

3.3

L

p-DIPB

473

19.0

32.0(m)

45.0

20.0

2.3

74.9(p)*

2.0

8.9

*p: para; m: meta




230

It is interesting to note that E M T and F A U give different results. Since the only difference between these two samples is the structure, clearly in this case we are observing the difference in structure-property relationships. Because the amount of the isomerized product is so high for EMT, one must be careful not to overinterpret these differences. Further syntheses and reaction experiments with F A U and E M T will be performed in order to verify these results. Also, one might expect similar data from F A U and SAPO-37. Since SAPO-37 contains no alkali, the supercage will be slightly larger in SAPO-37 than in F A U . This may be the cause of the differences in the 1,3,5/1,2,4-TIPB ratios. Table IV shows a comparison of indexes derived from various test reactions. It is clear that CI and CI* can not discriminate well between 12-membered ring molecular sieves while SI can. It is not clear at this time whether the SI will continue to increase in magnitude with larger pore materials. Also, as previously mentioned, the use of a bifunctional reaction may prove difficult for testing materials with low stability. This point may not be as important as we initially believed since we recently have shown that platinum can be impregnated into VPI-5 and n-hexane reacted at temperature as high as 800 K. The ratio 1,3,5/1,2,4-TIPB does increase with increasing pore/void size and does reveal a difference between 10-13 Â sized microporous materials and the mesoporous S1O2-AI2O3. Thus, with further refinements this ratio may prove useful for characterizing larger pore molecular sieves.

Table I V . Comparison of Indexes

Catalyst

CI

CI*

SI

SAPO-5

-

1.5

4.0

0.2

0.15

Offretite

3.7

1.8

5.0

1.1

0.83

Mordenite

0.4

1.8

7.8

1.5

0.54

Beta

0.6

1.4

18.9

1.8

0.71

EMT

-

-

-

2.3

1.20

FAU

0.3

1.2

20.8

2.5

3.60

L

-

1.0

17.2

2.9

3.02

SAPO-37

-

1.3

-

3.0

3.37

3.5

4.47

Si0 -Al 0 2

2

3

-

1,3,5-TIPB 1,2,4-TIPB

M


15. KIM ET A L


231

Conclusions The alkylation of meta-diisopropylbenzene with propylene over the acid form of molecular sieves is suggested as a new test reaction to characterize the effective pore size of larger (12 T-atom rings or above) molecular sieves. The ratio of 1,3,5- and I, 2,4-triisopropylbenzene formed at conversions near 25 % shows a strong correlation with the size of the pore/void space, i.e., the ratio increases with increasing size.

Acknowledgement We thank the Akzo America, Inc. for the financial support of this work.


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Selectivity in Catalysis - American Chemical Society

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